Conventionally, a bearing device which supports a rod-shaped shaft such that the shaft is relatively rotatable around its central axis is used in various connections of a construction machine. Such a bearing device is formed in a cylindrical shape. A shaft is inserted in a bearing hole formed inside the bearing device to be relatively-rotatably supported. When the shaft relatively rotates, a thin oil film is formed by lubrication oil supplied between an outer surface of the shaft and an inner surface of the bearing hole. In fact, the shaft relatively rotates on the oil film of the lubrication oil.
By the way, according to a working characteristic of an excavator representing a construction machine, because the shaft reciprocally rotates a predetermined angle under a low speed and a heavy load, a relative slip speed of the outer surface of the shaft to the inner surface of the bearing hole is enormously slow, constantly causing a enormous load to a contacting area (contact angle area). Therefore, a local break of the oil film occurs between the outer surface of the shaft and the inner surface of the bearing, thereby causing seizure between the outer surface of the shaft and the inner surface of the bearing.
Consequently, a bearing device, which has a oil supply groove supplying lube oil to the inner surface of the bearing and has a ratio of 30 to 90% of an area of the oil supply groove to an area of the inner surface of the bearing, is proposed. In such a bearing device, lube oil is stably supplied through the oil supply groove, and a seizure resistance is improved by preventing from the local oil film break (for example, see Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-84815